Recently, chip type electronic parts have been developed in compliance with such requirements as miniaturization of electronic equipment and speedup in mounting onto a printed circuit board. Also, the development of a chip type electrolytic capacitor has been greatly demanded and thus various kinds of developments have been proposed.
However, in electrolytic capacitors, especially those using a liquid electrolyte, sealing of the liquid electrolyte in a given space is necessary. In general the sealing is achieved by placing a sealing substance made of elastic rubber onto an end-close cylindrical sheathing case including the capacitor elements.
In the case of miniaturization of the electrolytic capacitor with such a sealing structure, the sealing structure should be also miniaturized. In this case, the capacitor should be provided with a sealing means and a given space for placing the sealing substance in order to keep sufficient sealing effect, resulting in difficulty in the miniaturization of the capacitor. Accordingly, although various kinds of the chip type electrolytic capacitor have been developed for the miniaturization of an electrolytic capacitor body, the size of the capacitor is not less than about 4 mm to 10 mm in height from the printed circuit board. Therefore, it is extremely difficult to manufacture chip-type electrolytic capacitor of a size ranging from about 1 mm to 3 mm in height, which is substantially the same as that of a ceramic capacitor.
On the other hand, solid electrolytic capacitor using no liquid electrolyte generally comprise an anode plate made of a metal such as tantalum on which an oxide film layer is formed, a solid electrolyte layer made of a metal such as manganese dioxide which is formed on the anode plate, and a conductive layer made of carbon paste and silver paste or the like.
The thus-constituted solid electrolytic capacitor can be readily miniaturized in the chip-type form due to the solid electrolyte contained therein.
However, the capacity of a conventional solid electrolytic capacitor is limited to a range from about 0.1 to 10 microfarad. In addition, its impedance characteristic is superior to that of a capacitor using the liquid electrolyte but inferior to that of a ceramic capacitor. Moreover, if tantalum is used as the anode plate, the cost of manufacturing the capacitor becomes high.
In recent years solid electrolytic capacitor utilizing organic conductive compounds such as tetracyanoquinodimethane (TCNQ) and polypyrrole has been proposed. For instance, there are proposed solid electrolytic capacitors using polypyrrole as disclosed in the Japanese patent laid-open publications Nos. 63-158829, 63-173313, 1-228122, 1-232712, 1-251605, 1-243510, 1-260809 and 1-268111.
The solid electrolyte used for these solid electrolytic capacitors has higher conductivity than the conventional one consisting of metal oxide semiconductor. Therefore, such solid electrolytic capacitors have a high-frequency impedance characteristic and do not require sealing of liquid electrolyte in the capacitor body, so that the capacitors can be readily miniaturized.
However, the TCNQ complex is chemically unstable and specially inferior with respect to heat resistance. In some cases, an electrolyte layer of the TCNQ complex formed on an aluminum anode plate of the solid electrolytic capacitor deteriorates due to soldering heat ordinarily rising up to about 260.degree. C. Accordingly, such a solid electrolytic capacitor is not suitable for use in a chip.
On the other hand, the solid electrolytic capacitor using polypyrrole as electrolyte has high heat resistance due to it polymer character and can be readily applied for use in a chip type capacitor.
The polypyrrole layer is produced on the surface of the anode plate by chemical, electrolytic and vapor-phase polymerizations and the like. The polypyrrole layer per se does not have a great mechanical strength, so that it is sometimes damaged due to such mechanical stress as torsion and pressing force exerted on the anode plate used as a substrate.
Upon surface-mounting onto the printed circuit board, ordinary chip-type electronic parts are delivered and placed thereon by a jig such as a suction nozzle. In this case, it is known that the parts are loaded with about 1 kg pressuring force of the suction nozzle. The ordinary electronic parts have on their surface a resin coating sufficiently resistant against the load of about 1 kg. On the other hand, the miniaturized electronic parts of a thin type have an electrolyte layer of polypyrrole which has less mechanical strength so that it is susceptible to damage by the pressing force of the suction nozzle.
In addition, since polypyrrole is characteristically deteriorated by moisture, a surface coating having an improved moisture resistance is required.
These disadvantages can be eliminated by applying a thick resin coating onto the surface of the electronic parts while forming a polypyrrole layer on a rigid anode block, as employed in the conventional solid electrolytic capacitor. However, in that case the miniaturization of the whole part, namely the manufacture of the part having substantially the same size as that of a ceramic capacitor, is not achieved.